This invention relates to hydrogen-activated catalyst compositions comprising iron, silicon and carbon or iron and silicon that provide high catalytic activity and high selectivity to C.sub.2 -C.sub.6 alkenes in CO/H.sub.2 Fischer-Tropsch reactions.
Reaction of carbon monoxide and hydrogen mixtures, such as are available from gasification of coal, in the presence of a nickel, cobalt or iron catalyst with a suitable carrier or promoter at a temperature of 150.degree.-450.degree. C. and a pressure of 10-200 kPa atmospheres to produce liquid hydrocarbons is known as the Fischer-Tropsch process. See for example P. Biloen et al., Advances in Catalysis, Vol. 30, pp. 165-216 (1981), D. L. King et al., Catal. Rev. - Sci. Eng., Vol. 23, pp. 233-263 (1981) and Chem. and Eng. News, Oct. 26, 1981, pp. 22-32.
The Fischer-Tropsch process for production of hydrocarbons from carbon monoxide/hydrogen gas mixtures includes the following reactions: ##STR1## With many catalysts, water formed in the above reactions is easily converted to carbon dioxide via the water-gas shift reaction: EQU CO+H.sub.2 O.fwdarw.H.sub.2 +CO.sub.2 ( 5)
At high temperatures, carbon monoxide is also converted to carbon: EQU CO+H.sub.2 .fwdarw.C+H.sub.2 O (6) EQU 2CO.fwdarw.C+CO.sub.2 ( 7)
Even though the Fischer-Tropsch process has been long known and considered as a potentially useful process for manufacture of chemical feedstocks, especially hydrocarbons, unfortunately, most Fischer-Tropsch catalysts materials used to generate hydrocarbons produce a product mixture containing both alkenes and alkanes encompassing a broad range of molecular weights. Reactions of CO/H.sub.2 that produce predominately methane (methanation) or carbon dioxide are undesirable. This lack of selectivity makes these prior art Fischer-Tropsch processes uneconomical for large-scale production of hydrocarbons. Improved selectivity is being actively sought by addition to the catalyst, e.g., iron or cobalt of two types of promoters (1) metal oxides, e.g., alumina and (2) energetic promoters, e.g., alkali metal carbonates.
In particular, the improved selectivity for production of light olefins (C.sub.2 -C.sub.4 and C.sub.2 -C.sub.6) is considered highly desirable. In addition, it would be desirable to limit the amounts of CO.sub.2 and H.sub.2 O produced in reactions 5-7 of the Fischer-Tropsch process. Not only does removal of CO.sub.2 require a substantial cost in energy and equipment but CO.sub.2 and H.sub.2 O also are thought to drastically reduce the service lifetime of the Fischer-Tropsch catalyst.